An increasing interest is developing to incorporate both free-space optical and radio frequency (RF) transmission and reception capabilities into communication and sensor systems. For example, backup RF capabilities for use during inclement weather are being considered for modern optical communication systems. Also, airborne and air-to-ground systems generally require RF capability to operate through clouds. Likewise, certain sensor systems may be required to operate at both RF and optical frequencies, such as a sensor suite that includes both electronic warfare (EW) sensors operating at optical frequencies and a radar system operating at RF.
Such systems may require precise beam pointing over a wide range of pointing angles (i.e., a field of regard) for optimum performance and generally require high reliability, small size, and lightweight design. To date, gimbaled systems typically have been used to meet these requirements. Such systems move the entire optical system and RF system in azimuth and elevation to transmit and receive in a certain direction. To provide the required pointing accuracy, such systems tend to be rather large, heavy, and complex. This complexity in turn makes high reliability difficult and expensive to achieve.
Furthermore, such systems are not well suited for mounting onto an aircraft. Typical gimbal-based systems will either introduce excessive amounts of turbulence when mounted externally of an aircraft hull or will have a substantially restricted field of regard if placed within the airframe. Risley prism systems can be useful in these situations. By placing a set of prisms such that the outermost prism sits just within a conformal window on the aircraft, a communications terminal can have a large range of pointing angles while introducing virtually no turbulence onto the airframe.
However, existing prism systems, such as Risley prisms, generally cannot simultaneously transmit or simultaneously receive signals at RF and optical wavelengths in the same direction with one prism assembly. This is because materials from which prisms are typically formed tend to exhibit different indexes of refraction at different wavelengths (this phenomenon can be observed in the well-known context of a prism creating a rainbow of colored light due to a refractive index gradient across the visible spectrum). While existing Risley prisms could theoretically be used to transmit or receive both RF and optical signals, signal beams at these two wavelengths could not be steered simultaneously in the same direction. Rather, two initially co-linear beams having different wavelengths would exit the prism system at different angles and diverge as they travel through free space. Thus, two prism assemblies would be required to separately handle RF and optical signals, making the overall system more complex, bulky, heavy, and expensive. In the airborne context, this would lead to the RF and optical assemblies requiring two respective conformal windows or time sharing a single conformal window.